Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual é a média de importações estrangeiras de pedra quando o total de pedra é 3.157.247 e as exportações estrangeiras de pedra são inferiores a 358.493?
 ###Tabelas: CREATE TABLE 2008_draft_picks (2008_draft_picks_id AUTO_INCREMENT PRIMARY KEY, Round INT, Overall INT, Player VARCHAR, Position VARCHAR, Nationality VARCHAR, Club team VARCHAR) CREATE TABLE record_vs._big_12_opponents (record_vs._big_12_opponents_id AUTO_INCREMENT PRIMARY KEY, Oklahoma vs. VARCHAR, Overall Record VARCHAR, at Norman VARCHAR, at Opponent's Venue VARCHAR, at Neutral Site VARCHAR, Last 5 Meetings VARCHAR, Last 10 Meetings VARCHAR, Current Streak VARCHAR, Since Beginning of Big 12 VARCHAR) CREATE TABLE annual_tonnage (annual_tonnage_id AUTO_INCREMENT PRIMARY KEY, Year INT, U.S. Rank INT, Total s Ton INT, Domestic s Ton INT, Foreign Total s Ton INT, Foreign Imports s Ton INT, Foreign Exports s Ton INT) CREATE TABLE performance_series (performance_series_id AUTO_INCREMENT PRIMARY KEY, Model VARCHAR, Release date VARCHAR, Sensor res., size VARCHAR, Lens(35mmequiv.) zoom, aperture VARCHAR, Screen size,pixels VARCHAR, Dimensions W×H×D (mm) VARCHAR, Weight VARCHAR) CREATE TABLE by_electorate (by_electorate_id AUTO_INCREMENT PRIMARY KEY, Region VARCHAR, Enrolled men INT, Enrolled women INT, Enrolled total INT, Men of voting age INT, Women of voting age INT, Voting age population INT, E/VAP ratio Men VARCHAR, E/VAP ratio Women VARCHAR, E/VAP ratio Total VARCHAR)  
SELECT AVG Foreign Imports s Ton FROM annual_tonnage WHERE Total s Ton = 3,157,247 AND Foreign Exports s Ton < 358,493